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Initial Fault Trees

The top of the tree, read off the output ports.

The first level maps the malfunction onto the failure cases of the system's output signals — and only those — joined by OR where any one suffices and by AND where several must coincide. Because each event is generated from the ibd, it carries a complete structured address: its kind, location, exact connection, fault mode, and rate. That is what later lets a cut set be a located combination of real faults.